1. Technical Field
The present disclosure relates generally to electric machines, for example, permanent magnet motors and generators.
2. Description of the Related Art
Electric machines, such as electric motors and generators, are used in many applications, including those ranging from electric vehicles to domestic appliances. Improvements in machine performance, reliability, efficiency, and power density for all types of electric motors are desirable.
An electric machine converts electrical or electromagnetic energy into mechanical energy or conversely converts mechanical energy into electrical or electromagnetic energy.
The permanent magnets used in rotor assemblies are disposed within axially extending pockets. The pockets are typically formed near the outer perimeter of the rotor hub, which is built up from laminations made from electric grade steel. Electric grade steel is used on rotor assemblies because it has a greater permeability for conducting the magnetic lines of force. The process of building up a rotor with laminations is done to reduce eddy current losses in the rotor hub, especially during higher rotation speeds. The rotor extends from its outer perimeter to an inner diameter that interfaces with a shaft. The total mass of the rotor assembly is one of the parameters that affects the acceleration characteristics of the electric motor, the cost of the rotor assembly, and the amount of stress experienced by the various components of the rotor assembly, among other things.
Shafts used in electric machine are typically made from structural steel, which is slightly more dense and certainly stronger than electric grade steel. In one application, an electric motor of the Toyota Prius, which is a hybrid vehicle, utilizes a hollow shaft with an integrated carriage. The carriage includes a central web having one end connected to the main shaft and the other end connected to a carriage support that extends axially in either direction away from the central web. A laminated rotor hub with permanent magnets is retained within the carriage support. The inclusion of the central web extending radially from the shaft creates unique balancing issues with respect to vibration modes. The bearing positions on the shaft of the Toyota Prius shaft must be positioned to minimize the bending stress arising from the central web. Thus, although the Toyota Prius shaft provides dome marginal weight reduction benefits, the configuration of the rotor assembly is not readily convertible to other types or sizes of motors.
Conventional rotor assemblies include rectangular-shaped rotor pockets in which the rectangular-shaped permanent magnets are disposed. In these conventional rotor assemblies, the stress concentrations in the magnet pockets and in the rotor laminations exacerbate the localized stresses as the operating speeds increase. When the rotor rotates at high speeds, the permanent magnets exert an outward radial force on the magnet pockets, which results in the centrifugal forces being reacted at the outer corners of the pockets. These localized stresses in conventional rotor assemblies are one reason for providing more material in the rotor.
It would be desirable to reduce the mass of the rotor hub, the shaft, and the permanent magnets either individually or collectively while maintaining a rotor assembly configuration that could be easily manufactured and scaled to different size electric machines.